The present invention relates to an optical hybrid integrated device having such semiconductor optical elements as a laser diode and a photodiode and an optical waveguide mounted on its substrate and a method of making such an optical waveguide device.
The optical hybrid integrated device is a device of the type in which there are mounted on the same substrate desired elements such as a light emitting element, a photodetector, an optical modulator, an optical filter, a wavelength shifter, an optical waveguide and an optical coupler. Furthermore, one end portion of an optical fiber, for instance, is positioned in and fixed to a V-groove cut in a silicon substrate with the end face of the optical fiber facing toward one end face of the optical waveguide for connection with another optical device. For example, the active layer (a light emitting region) of the laser diode is several micrometers above its electrode; this vertical position is appreciably lower than that of a core of the optical waveguide to which the laser diode is to be optically connected. The core is usually about 10 .mu.m thick and buried in an about 40-.mu.m thick clad layer of the optical waveguide substantially centrally thereof. Accordingly, in the case where the clad layer with the core buried therein is formed on the silicon substrate and the laser diode with its active layer underside is disposed on the same substrate surface in opposing relation to the end face of the core, the vertical positions of the optical waveguide (core) and the active layer of the laser diode are greatly displaced from each other. An optical hybrid integrated device manufacturing method which solves this problem is described, for example, in Horiguchi, "Hybrid Optical Integration Techniques," Denshi Zairyou, pp.97-102, June, 1995.
With reference to FIGS. 1A through 1D, the proposed manufacturing process will be described below in brief. To begin with, a terrace 10A is formed by etching in the surface of a silicon substrate 10 and then an under-clad glass layer 2A of a quartz optical waveguide is formed over the entire area of the substrate surface as depicted in FIG. 1A. Then, the under-clad layer 2A is ground until the top of the terrace 10A is exposed, that is, the terrace 10A is surrounded by the under-clad layer 2A, after which a height adjustment layer 2B of the same material as that for the clad layer is formed all over the under-clad layer 2A including the terrace 10A as depicted in FIG. 1B. Then, a core 2C which serves as an optical waveguide is formed by patterning, and an over-clad layer is formed all over the substrate surface as shown in FIG. 1C. After this, the over-clad layer is selectively etched away to exposed the top surface of the terrace 10A as depicted in FIG. 1D. A semiconductor optical element 3, such as a laser diode or photodiode, is mounted on the terrace.
The thickness of the height adjustment layer 2B is predetermined so that the core 2C lies at the same vertical position as that of an active layer 3A of the optical element 3. According to this method, the vertical positioning of the active layer 3A of the semiconductor optical element 3 with respect to the optical waveguide (core) 2C need not be performed at the time of mounting the semiconductor optical element. Since an optical waveguide, an optical element, an optical fiber, and so forth are usually mounted on a single substrate, the fabrication procedure is complex and it is difficult to increase the packaging density.
A method which facilitates the manufacture of the optical hybrid integrated device and provides increased packaging density is proposed, for example, in Japanese Patent Application Laid-Open Gazette No. 10-133069, according to which a second substrate with an optical waveguide formed thereon is loaded on a first substrate with a semiconductor optical element mounted thereon, by positioning them using alignment marks formed thereon at corresponding positions. With this method, the depth of a V-groove for fixing therein an optical fiber can be predetermined so that the core of the optical fiber and the optical waveguide are at the same vertical position.
According to this method, for example, as depicted in FIG. 2A, there are formed alignment marks 18 and solder-coated electrodes 15A and 16A in the surface of arectangular silicon substrate 10 which has V-grooves 11A and 11B cut therein. Further, as shown in FIGS. 2B and 2C, there are provided on the surface of a second substrate 20 a clad layer 21 and a core 22 buried therein and forming optical waveguides 22a and 22b. On the surface of the second substrate 20 there are formed marks 24 corresponding to the alignment marks 18 on the first substrate 10. By accurately maintaining the positional relationships of the alignment marks 18 to the V-grooves 11A, 11B and the solder-coated electrodes 15A and 16A, the second substrate 20 is mounted on and soldered to the first substrate 10 with the clad layer 21 on the underside with the alignment marks held in position.
In this optical hybrid integrated device, the depths of the V-grooves 11A and 11B are predetermined taking account the thickness of a solder layer and the height from the clad surface of the optical waveguide to the core so that the vertical position of the core of the optical fiber is the same as that of the optical waveguide (core) relative to the top surface of the substrate 10. As regards the heights of the active layers of a light receiving element and a light emitting element which are fixed on the solder-coated electrodes 15A and 16A on the substrate 10, the thickness of an metal electrode on which a solder layer is formed is predetermined so that the vertical positions of the light receiving face and the light emitting face of the optical elements are the same as the vertical position of the optical waveguide (core). However, it is difficult that the solder-coated metal electrodes having thicknesses of several to tens of micrometers are formed with tolerances of 1 .mu.m or better. This problem could be solved by a method in which four or two pedestals are formed to desired thicknesses on a substrate with tolerances of 1 .mu.m or better in correspondence to four corners or both sides of the bottom the light receiving element or light emitting element except its central area, the light receiving element or light emitting element is mounted on the pedestals and an electrode on the substrate and the light receiving element or light emitting element is connected by a solder bumps midway between the four or two pedestals. However, this method has the defect of increasing manufacturing steps for forming the pedestals.